Among conventional nitride semiconductor devices is one fabricated by performing oxygen plasma processing on a surface of an n-type GaN contact layer to form an oxygen-doped layer and thereafter forming an ohmic electrode on the n-type GaN contact layer so that a contact resistance between the n-type GaN contact layer and the ohmic electrode is reduced (see JP 2967743 A (PTL1)).
However, with respect to the above nitride semiconductor device, when the present inventor actually performed experiments to form an ohmic electrode after performing oxygen plasma processing on the GaN layer, the ohmic electrode showed a high contact resistance and sufficiently low contact resistance was by no means able to be obtained.